In order to protect a vehicle occupant from impact, use of a vehicle airbag has recently been increased.
Such an airbag is inflated instantaneously when a vehicle collision is detected, and its operation depends mainly on the performance of an impact sensor.
U.S. Pat. No. 4,827,091 discloses an acceleration sensor designed to detect a substantial change in the speed of a motor vehicle in the event, for instance, of a vehicle collision and comprises a cylindrical body formed of an electrically conductive material, a magnetic inertial mass mounted for axial movement within the cylindrical body, an electrically conductive element attached to at least one axial end of the magnetic inertial mass, a pair of electrical terminals mounted to one axial end of the cylindrical body and electrically bridged upon contact with the electrically conductive element of the magnetic inertial mass, and a magnetic biasing body mounted to the other end of the cylindrical body and adapted to magnetically bias the magnetic inertial mass.
With this acceleration sensor, the magnetic inertial mass is magnetically biased toward the magnetic biasing body. When the acceleration sensor is subject to no or little accelerating force, the magnetic inertial mass is fixed in place at the other end of the cylindrical body.
When the acceleration of sufficient magnitude is applied, the magnetic inertial mass is displaced against the biasing force of the magnetic biasing body. At this time, an electric current is induced in the cylindrical body to provide a magnetic force to return the magnetic inertial mass or to retard the displacement of the magnetic inertial mass. This results in a descease in the speed of the magnetic inertial mass.
When the acceleration input is less than a predetermined (threshold) level, the magnetic inertial mass does not reach the one end of the cylindrical body and then returned to the other end of the cylindrical body from any other position short of the one end of the cylindrical body by a biasing force of the magnetic biasing body.
When the acceleration input is greater than a predetermined (threshold) level (for example, a vehicle incorporating this acceleration sensor is crashed), the magnetic inertial mass reaches the one end of the cylindrical body. The electrically conductive element or end surface of the magnetic inertial mass is then brought into contact with the both terminals to electrically bridge the two terminals. If a voltage has previously been applied between the two terminals, an electric current flows between the terminals when the two terminals are short-circuited. This electric current shows a vehicle collision.
In the sensor of U.S. Pat. No. 4,827,091, the magnetic inertial mass is advanced to a full extent so as to contact a pair of electrodes. This electrically bridges the two electrodes. In order for the sensor to properly operate for a prolonged period of time, the inner suface of the cylindrical body within which the magnetic inertial mass is slidably moved must be totally flat and free of friction and corrosion. Additionally, the ends of the electrodes and the magnetic inertial mass must be totally free of rust or corrosion. Therefore, the cylindrical body should be made of a material which provides high resistance to corrosion, and its inner surface should also be finely abraded. This results in a substantial increase in the production cost. Also, the electrodes should be plated with gold so as to fully prevent corrosion. This further increases manufacturing costs.